1. Field of the Invention
The present invention relates to a track servo control method, a track servo controller and an optical storage device for performing follow-up control of an optical beam to a track of an optical disk, and more particularly to a track servo control method, a track servo controller and an optical storage device for accurately following up an optical beam to a track of an optical disk where the ID part of the sectors are embossed.
2. Description of the Related Art
The storage capacity of an optical storage device using an optical storage medium, such as an optical disk (disk type, card type), has been improved dramatically. In an optical storage device, an optical disk storage medium having spiral type or circular type tracks is used, an optical beam of an optical head is positioned on a track, the optical beam is focused, and data on the track is read or read/written by the optical beam. A storage device where data is magnetically recorded on a magneto-optical disk by the optical beam is also defined as an “optical storage device”.
For this, a track servo control for following up the optical beam to the track and a focus servo control for following up the focal position of the optical beam to the recording face of the medium are performed. For these servo controls, a focus error signal (FES) for indicating the displacement amount in the focus direction, and a track error signal (TES) for indicating the displacement amount in the track direction, are generated from the reflected light of the optical beam from the medium, and feedback control is performed so that the displacement amount becomes zero.
In such a focus or track servo control method, periodic noise may be applied to the control system. For example, as FIG. 32 shows, the optical disk medium 200 is divided into a plurality of zones 200-1 and 200-2 in the radius direction and in each zone, and a plurality of tracks 201 are formed in parallel. These tracks 201 are divided into a plurality of sectors in a circumferential direction of the medium, and a header 202, to indicate a sector mark and ID (track number, sector number), is provided at the beginning of each sector.
In the optical disk 200, this header 202 is embossed in a relief structure or in a geometric shape (here this is referred to as “pre-pit”). In other words, the header 202 is comprised of convex protrusions or concave holes formed as a geometric shape. This method allows creating a header 202 of an optical disk comprised of tens of thousands of tracks and hundreds of thousands of sectors by stamp-processing in an optical disk substrate manufacturing step, and does not require writing, as is required for a magnetic disk.
The header 202 is mechanically formed with convex/concave pits, so when the optical beam 210 travels on a track 201 of such an optical disk 200, the track error signal (TES) to be input to the positioning system changes, and the track error amount increases by the header 202 of the sector of the track 201. In other words, noise is applied to the positioning system. Since this noise depends on the header position, this noise is regarded as periodic noise.
One general countermeasure to the noise applied to the control system is decreasing the gain of the closed loop characteristic of the control system at the frequency where the noise is generated. The closed loop characteristic is given byPK/(1+PK)  (1)
where P is the characteristic of the control target and K is the characteristic of the controller. Since this is a response characteristic of a position error by the noise, the influence of the noise is decreased by dropping the gain of the closed loop characteristic with an improvement of the characteristics of the controller.
A method for decreasing the noise of the ID pit (header) of a magneto-optical disk has been proposed in Japanese Patent Laid-Open No. H5-258383. As mentioned above, in a magneto-optical disk, a track on the medium is divided into small areas called “sectors”, and a part recording a track number and sector number to indicate a position on the disk, called the “header (ID part)”, is at the beginning of a sector.
This part is formed to be a pre-pit when the medium is created, and is comprised of physical dots. Therefore if the laser spot passes over the ID part, light is diffused and the quantity of reflected light decreases, which is observed as noise. According to the method in Japanese Patent Laid-Open No. H5-258383, the ID part detection unit detects the timing when the laser spot passes over the ID part, so that the control circuit is switched only at this time, to prevent the vibration of the actuator by noise.
A problem of the conventional method of decreasing the gain of the closed loop characteristic is the deterioration of the follow-up performance which frequently occurs. The closed loop characteristic has a close relationship with the sensitivity characteristic, which is the disturbance suppression characteristic, and this relationship is given by T+S=1, where the closed loop characteristic is T, and the sensitivity characteristic is S. Therefore the closed loop characteristic cannot be changed independently from the sensitivity characteristic, and the sensitivity characteristic often deteriorates by decreasing the gain subject to the frequency of the noise. As a result, the follow-up accuracy deteriorates even if the influence of the noise can be decreased.
In the case of the method stated in Japanese Patent Laid-Open No. H5-258383, the ID part detection unit can detect only the ID part of the zone where the optical beam is following up at that time. So the ID pit noise of the zone during follow-up can be removed quite well.
On the medium of a magneto-optical (optical) disk, tracks are curved as spirals, and several hundred to several thousand tracks are grouped in units called “zones”. In some types of medium, the number of ID parts differs depending on the zone, and as FIG. 32 shows, the position of the ID part is different depending on the zone, as shown in zone 200-1 and zone 200-2. Therefore in the case of a high density optical (magneto-optical) disk, not only the ID parts of the zone being followed up but the ID parts of the peripheral zones thereof as well may be observed as noise with the track error signal (TES).
For example, FIG. 33 shows the progress of the run-out amount with respect to the time during track follow-up control of the high-density (e.g. track width is less than 1 micron) magneto-optical disk. As FIG. 33 shows, other noise which periodically occurs exists in addition to the noise generated by the ID pits of a currently following up track which appears periodically.
The noise generated by the ID pits of a currently following up track can be removed by the above mentioned conventional ID pit noise removal method, but other periodic noise cannot be removed, since the cycle is different. Especially when the track density is high as in the above mentioned case, the allowable positional error during track follow-up is small, so the influence of the above mentioned noise cannot be ignored.